The present invention relates generally to a stylet safety shield.
Needle assemblies have particular, although not exclusive application in the field of medicine and have tubular needles with sharpened ends for use in piercing the skin to withdraw materials as needed. The needle is supported by some other structure that is used to manipulate the needle. The most common example is a syringe. However, some needle assemblies require the application of substantial force in use. One example of such a needle assembly is a bone marrow needle assembly that is used to penetrate cortical bone to reach the intramedullary canal for withdrawing liquid and/or a biopsy sample of bore marrow, or for infusing the canal with a selected material. Typically, the needle includes a cannula and a stylet that is received in the cannula and has a hard, sharp tip that can penetrate cortical bone. The tip projects out from the distal end of the cannula. The stylet can be withdrawn from the cannula after the needle penetrates the bone so that the hollow interior of the cannula can be used as a conduit for liquid or a receptacle to collect bone marrow.
In order to penetrate cortical bone, a substantial amount of force must be applied to the needle. For this reason, bone needle assemblies conventionally mount the needle in a handle that is sized and shaped so that the technician may comfortably grip the handle and apply the force necessary to penetrate the bone. The handle may comprise a stylet handle member 16 that mounts the stylet and a cannula handle member 18 that mounts the cannula. The two handle members are removably securable together to form the handle when the stylet is inserted into the cannula so that the stylet handle member 16 is in contact with the palm of the technician's hand in use, and the cannula handle member 18 receives the technician's fingers.
Some needle assemblies, including bone needle assemblies, have associated safety mechanisms that shield the sharp tips of the needle components when they are not needed and after they have become contaminated with potentially hazardous biological material. For example, one conventional type of bone needle assembly 10 includes a safety shield slidably received on the stylet for shielding the tip of the stylet. When the stylet is received in the cannula and the stylet handle member 16 is secured to the cannula handle member 18, the stylet safety shield is releasably secured within a cavity of the cannula handle member 18 and is housed within an enclosure defined by the cavity in the cannula handle member and a cavity in the stylet handle member. The stylet safety shield remains secured to the cannula handle member 18 when the stylet handle member 16 is removed from the cannula handle member and the stylet is withdrawn from the cannula. The stylet slides through the stylet safety shield as the stylet is withdrawn from the cannula until the mechanism in the safety shield fixedly engages the stylet at the stylet tip. As the technician continues to pull the stylet handle member 16 away from the cannula handle member 18 after the shield fixedly engages the stylet, the safety shield disengages the cannula handle member, and the stylet with the shield disposed over its tip is separated from the cannula.
Although the conventional stylet safety shield design described above is an easy, passive mechanism that ensures that the tip of the stylet will be covered after use and removal from the cannula, drawbacks have been realized and identified by the applicants. For instance, a desired bone marrow sample may not be retrieved on the first attempt, requiring reuse of the needle assembly. If the shield is automatically locked over the tip, it will be very difficult to expose the tip again for use. Moreover, in attempting to re-expose the tip, the danger of a stick is greatly magnified. Thus, the passive shield may lead to the very thing it was supposed to prevent.